drm/mediatek: mtk_dpi: Use switch in mtk_dpi_config_color_format()

drm/mediatek: mtk_dpi: Use switch in mtk_dpi_config_color_format()

In preparation for adding support for an additional color format,
convert the format conditional in mtk_dpi_config_color_format()
to a switch.
This also makes the concept of RGB being the default color format
a little more human readable.

This commit brings no functional differences.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250409131306.108635-2-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>